1. Field of the Invention
The present invention relates to a fuel injection quantity control device for an alcohol engine, which device is designed to switch a fuel-injecting mode according to a fuel injection pulse width variably arranged based on an alcohol density.
2. Description of the Related Art
Recently, there has been developed an alcohol engine dedicated for a flexible fuel vehicle (FFV) (referred to as an FFV alcohol engine) in order to meet the requirement for reduction of fuel consumption or purification of exhaust air. The alcohol engine can be driven by not only a gasoline fuel but also an alcohol fuel or a mixed fuel consisting of gasoline and alcohol.
The FFV alcohol engine is normally controlled to correct a fuel injection pulse width according to an alcohol density contained in the used fuel for keeping a theoretical air/fuel ratio. The alcohol density is sensed by an alcohol sensor. One example of the FFV alcohol engine is disclosed in the Official Gazette of Japanese Laid-open No. 58-28557.
However, it has turned out that the theoretical air/fuel ratio sensed in case of using 100%-alcohol fuel is about half as high as that sensed in case of using a 100%-gasoline fuel. As the alcohol density becomes higher in the fuel, therefore, the theoretical fuel/air ratio becomes lower.
In the FFV alcohol engine, hence, the fuel injection quantity given in case of using a 100%-alcohol (0%-gasoline) fuel is about twice as large as that given in case of using 0%-alcohol (100%-gasoline) fuel even under the same driving condition.
If a normal injector nozzle diameter is expanded to double the fuel injection quantity, the alcohol engine entails a shorter fuel injection pulse width, that is, valve-opening time in a low-load engine driving area such as idling drive when the engine uses the 0%-alcohol (100%-gasoline) fuel. It results in the fuel injection quantity of the injector showing variations in fuel injection quantity and worsening a fuel atomization characteristic, thereby making the engine combustion unstable.
If, on the other hand, the normal injector nozzle is not improved, the alcohol engine is required to have a doubled fuel injection pulse width when it uses the 100%-alcohol fuel. Assuming that it is a four-cycle engine, the normally-feasible maximum fuel injection pulse width T.sub.MAX matches to a one-cycle (720.degree. CA) passage time, that is, T.sub.MAX =(60/N).times.2 sec. It means that the actual maximum fuel injection pulse width T.sub.MAX is made shorter than that, because it is necessary to consider the fuel injection timing at the next cycle.
The maximum fuel injection pulse width T.sub.MAX, however, is variable on the number of revolutions of an engine. In the driving area where the engine is subject to high rotation and load, thus, the engine may be short of the fuel injection quantity even if the injector is fully opened, thereby lowering its output and driveability.
The FFV alcohol engine requires a wide dynamic range from a minimum fuel quantity consumed in the idling drive with a 100%-gasoline fuel to a maximum fuel injection quantity consumed in the maximum with a 100%-alcohol fuel. Actually, the dynamic range required in the FFV alcohol engine is about twice as wide as that required in the normal engine. There exists large difficulty in developing such an injector to satisfy the requirement. The developed injector results in being a special product, thereby greatly raising the overall cost of the engine.